1. The Field of the Invention
The present invention is in the field of dental bleaching compositions, devices and methods for whitening a person's teeth. The invention relates to dental bleaching compositions having a high level of rheological stability, stickiness, low stringing tendency, and high internal cohesiveness.
2. The Relevant Technology
To achieve whiter teeth, people often have their teeth covered with veneers or chemically bleached. A conventional in-office bleaching process generally involves: (1) making an alginate impression of the patient's teeth; (2) making a stone cast or model of the impression; (3) vacuum forming a dental tray from the model, usually from a heated sheet of thin ethyl vinyl acetate (EVA) material, (4) trimming the upper tray rim to exclude gingival coverage, (5) introducing a bleaching gel into the tray, and (6) placing the tray over the teeth to be bleached for a suitable time period to effect tooth bleaching.
Because of the time and cost associated with forming customized trays, less time consuming and costly alternatives have been developed. Examples include boil and bite trays and non-custom trays, both of which tend to bulky and uncomfortable, flexible strips, which have a tendency to slip off the teeth, and paint-on bleaching compositions, which are directly exposed to saliva in a person's mouth.
In order to adhere a dental bleaching composition to a person's teeth, a tackifying agent is typically used. A common tackifying agent is carboxypolymethylene, an acidic polyacrylic acid polymer cross-linked with polyallyl sucrose. According to U.S. Pat. No. 6,500,408 to Chen, carboxypolymethylene allegedly causes tooth irritation because of (i) its ability to etch teeth because of its generally acidic nature and (ii) its tendency to chelate calcium ions found in teeth. In order to avoid such problems, Chen teaches the use of polyvinylpyrrolidone (PVP) polymers, which are neither acidic nor are believed to bind calcium. Besides carboxypolymethylene, Chen also disparages the use of fumed silica. Col. 3, lines 37-50. It is known that including fumed silica in an amount so as to adequately thicken and form a firm bleaching gel can greatly reduce the ability of a peroxide bleaching agent to bleach teeth. Fumed silica has an extremely high surface area, is highly hydrophilic and therefore binds strongly to water. By strongly binding to the water, fumed silica included in an amount sufficient to form a firm gel inhibits the passage of water into the tooth surfaces to be bleached. Because water is generally necessary to carry the peroxide bleaching agent into the tooth surfaces to be bleached, fumed silica that binds water indirectly inhibits passage of the bleaching agent into the tooth surfaces, greatly inhibiting tooth bleaching. For this reason, Chen teaches the desirability of using PVP polymers as the sole or primary thickening agent.
The problem with PVP and other linear polymers is they tend to form highly stringy dental bleaching compositions when included in an amount sufficient to yield a sticky, viscous composition that is able to adhere a dental tray to a person's teeth. This can greatly complicate the process of filling a dental tray with such compositions on a mass production scale. Moreover, even though compositions that include high amounts of PVP can be very adhesive, they may still lack sufficient thickness or viscosity to avoid running out of a pre-filled dental tray. Stickiness does not necessarily correlate with sufficient viscosity and yield stress to avoid running from a dental tray. A highly sticky Newtonian fluid may nevertheless be able to run out over time due to the force of gravity.
In general, the tendency of high potency dental bleaching compositions to run out of a pre-filled dental tray increases over time due to the tendency of polymeric tackifying agents such as carboxypolymethylene and PVP to lose viscosity and yield stress over time. This may be due to small but significant attack by the dental bleaching agent, typically a peroxide, on the tackifying polymer when included in high concentrations. Even though a dental bleaching composition may be sufficiently thick and viscous to avoid running out of the dental tray when first placed therein, it may nevertheless become runny over time, thereby critically reducing shelf-life in the case of over-the-counter products that must have a shelf life of several months or years. As a result, high shelf-like bleaching products typically have lower levels of peroxide to avoid viscosity breakdown and the tendency to become runny.
Although polymeric thickeners such as polysaccharide gums (e.g., xanthan gum) and carboxymethyl cellulose can be used to enhance the body and yield stress of dental bleaching compositions, they also create long-term stability issues. Such polymers tend to break down in the presence of highly concentrated peroxide bleaching agents. Worse, such polymers can react with peroxide bleaching agents such as hydrogen peroxide and form reactive organic peroxide species that can attack a person's gums and also the cross-linkages within carboxypolymethylene, which can greatly reduce its tackifying and thickening ability. Such thickeners are therefore only suitable for low level bleaching compositions containing 5% or less peroxide bleaching agent.
In view of the foregoing, there is a need to provide improved bleaching compositions that are sticky and viscous while having improved processing ability and long-term rheological stability. Such compositions would be particularly well-suited for use in manufacturing pre-filled dental bleaching devices (e.g., trays and strips).